Update Schematic to use Default Power Symbols

[michaelshipman]

There are pre-existing power symbols that exist by default in KiCad. This can provide improved clarity over custom labels. The library named power contains these, notably +3V3 and +5V.

Additionally, the most common convention I've seen in schematics is to orient positive voltage nets upwards and your ground net downwards rather than sideways. In my opinion this also makes the schematic easier to parse.